676 Professor Bewar [June 10, 



Electric Arc in different Gases, 



The experiments were made with a De Meritens dynamo-electric 

 machine, arranged for high tension, giving an alternating current 

 capable of producing an arc between carbon poles in air of from 8 to 10 

 millims. in length. The carbon poles used were 3 millims. in diameter, 

 and had been previously purified by prolonged heating in a current 

 of chlorine. This treatment, though it removes a large part of the 

 metallic impurities present in the commercial carbons, will not remove 

 the whole, so that lines of calcium, iron, magnesium, and sodium may 

 still be recognised in the arc. Besides the traces of metallic impuri- 

 ties, a notable quantity of hydrogen always remains unremovable by 

 this treatment with chlorine. 



The arc was taken in different gases inside a small glass globe {aa 

 in PI. I. Fig. 1) about 60 millims. in diameter, blown in the middle of 

 a tube. The two ends of the tube {hb) were closed with dry corks, 

 through which were passed (1) the carbons (cc), inserted through two 

 pieces of narrow glass tubing; (2) two other glass tubes {dd) through 

 which currents of the different gases experimented on were passed. 



The arc in the globe filled with air gave a tolerably bright con- 

 tinuous spectrum, on which the green and blue hydrocarbon bands 

 were seen, also the seven bands in the indigo (wave-lengths 4600 to 

 4502, Watts) as in the flame of cyanogen, and much more brightly the 

 six bands in the violet (wave-lengths 4220 to 4158, Watts) and five 

 ultra-violet. Besides these bands, lines of iron, calcium, and sodium 

 were visible. The arc in this case was practically taken in a mixture 

 of nitrogen and carbonic oxide, for in a short time the oxygen of the 

 air is converted into carbonic oxide. 



On passing through the globe a current of carbonic acid gas, the 

 bands in the indigo, violet, and ultra-violet gradually died out until 

 they ceased to be visible continuously, and when momentarily seen 

 were only just discernible. On the other hand, the hydrocarbon 

 bands, yellow, green, and blue, came out strong, and were even bril- 

 liant. Lines of iron and calcium were still visible. On stopping 

 the current of carbonic acid gas and allowing air to diffuse into the 

 globe, the violet and ultra-violet bands soon began to appear, and 

 presently became permanent and bright, the hydrocarbon bands 

 remaining bright. 



When a continuous current of dry hydrogen was passed through 

 the globe, the arc, contrary to what would be expected from the 

 behaviour of the spark discharge in hydrogen, would not pass through 

 more than a very short sj)ace, very much less than in air or carbonic 

 acid gas. There was a tolerably bright continuous spectrum, with no 

 trace of bands in the indigo, violet, or ultra-violet, and no metallic 

 lines, with the exception of a fairly bright line in the red, which 

 we identified, by comparison with the spark in a vacuum tube, with 

 the C line of hydrogen. The F line, identified in like manner, was 

 also seen as a faint diffuse band. This last line was in general 



